If the feedback does not amplify the CO2 warming, the warming for a doubling of CO2 will be about 1 °C. This is not disputed. The question is the amount of net positive feedback, and the amplitude of unrelated natural variability.

Yes, these are important questions and climate scientists have been working on them for a long time. I study astrophysics, but atmospheric dynamics is another academic interest of mine so I can go into some more detail here.

As I mentioned earlier, the uncertainty range for climate sensitivity is about 2 to 7 degrees C. This is a probability distribution, the peak of which is about +3°C. That means the 3° figure is much more likely than 2°, or 7°, so the agreement is much better than you might glean if you only read "somewhere between 2 and 7". The distribution is also non-symmetric, which explains why the maximum is not at 4.5 (half-way in-between). You can see the shape of the distribution here. Our understanding of this is not all that new, either. For example here is a paper from 1984 whose conclusion is in very good agreement with currently accepted figures. So you can see this has been studied for a while and our confidence in these figures is fairly good. That multiple different lines of evidence lead to very similar probability distributions is also an indicator that these results are robust.

When we apply our current understanding of climate sensitivity to our global climate models, and account for different possible emission scenarios, we get a distribution of future temperature changes. Given that humanity appears to be following the high emission (A2) scenario for the foreseeable future, we can expect to see continued warming. At this rate we can rule out a temperature change of less than 2°C by the year 2100. Recall, 2°C is what scientists generally agree is the maximum safe threshold for avoiding the most severe effects of climate change. We are already seeing the effects from only ~0.8° of warming. The upper estimate of 7° would be disastrous. So, this is a major issue and humanity should take it extremely seriously, despite the fact that there are non-zero uncertainties. Science is never without uncertainties. That's how you know it's good science.

Now you've mentioned other factors -- natural variability, solar activity, cloud feedbacks, etc. These are all important, of course. However, they do not significantly affect the results. First off, natural variability is well understood and there have been numerous studies on it. The IPCC has this classic image illustrating the various radiative forcings and their uncertainties. It's actually a bit dated now -- there have been many more recent studies. Here's a good example from 2011. In short, the natural variability is not very significant compared to the anthropic influences when considering the last century of climate change. The anthropic GHG emissions are both necessary and sufficient to explain the vast majority of the warming. Remove the CO2 and we can explain neither the temperature changes for human history nor for geologic history. (Aside: this is an excellent video presentation at AGU a few years ago on the role of CO2 in geologic history -- very highly recommended. It's about 45 minutes.)

You are completely correct about clouds -- their role as a climate feedback effect is surprisingly complicated and there are scientists who specialize purely on this subject. Without getting too involved in details, the understanding at this point is that for short time periods, the net radiative forcing from clouds is very small and possibly even positive, up to 0.5 watts per m2 per Kelvin. [source] There is a dependency on both temperature and time, and so for long-term future considerations the forcing becomes negative. However, it is not enough to cancel out the other positive feedbacks for realistic ranges of temperature.

The topic of solar activity on cloud formation and global temperature is one that is often misunderstood by the public -- generally we are dealing with how solar activity affects the upper atmosphere. Its effect on climate change in the lower atmosphere and Earth's surface is fairly small, which is why if you try to compare records of solar activity to temperature records, you get poor correlation. For surface temperatures over the last century, it is more of a fine-tuning knob then a major driver. Natural processes that more strongly impact surface temperatures for human timescales include El Nino / La Nina and semi-periodic atmospheric oscillations (eg the NAO), volcanic eruptions (which reduce temperature over short periods due to aerosols), etc. These factors are all discussed in the paper on temperature evolution from 2011 I linked above.

I'm under the impression that many astrophysicists downplay the CO2-temperature link and put much faith in the sun/cosmic ray link. But of course there might be some interest bias, where the importance of one's own interests gets exaggerated. That goes for astrophysics as well as for any scientist in another field. So if you have a foot in more than one discipline, good for you.

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Aside: this is an excellent video presentation at AGU a few years ago on the role of CO2 in geologic history -- very highly recommended.

Right. As I wrote previously, the number that is the answer to the sensitivity question is in my opinion less interesting scientifically than the reasons for that number. So even if the probability distribution actually nails it, we don't necessarily know the reasons. If you try to model something and it turns out that however you make the models, you get basically the same number out, then, yes, you get a really convincing probability distribution, but you will still have absolutely no clue what's going on in the physical world, since even false causes give the same result and you can't know that you picked the right model. If you're sure about the answer, you can't simply tune the knobs until you arrive at that answer and then claim that the science is settled, because everything points to your answer. That's the point I'm trying to make.

The geological aspect is interesting. There absolutely must be a beautiful thermostat. Otherwise we wouldn't have been around. Also, the geological record shows changes over centuries and millennia, while IPCC focuses on predictions for a shorter time scale, decades. To get decadal resolution no doubt there will be a lot of other factors that will not show up well in a millennial mean. The sun/cosmic ray thing might be one such thing. It's claimed that there is a link between the length of a solar cycle and the temperature in the next cycle, and, yes, there is some evidence for that, but there are many fallacies. Again, even if you know the answer, you probably can find a way to arrive at it and also probably be wrong unless you're very careful.

The best proof for the right sensitivity will come from observations, but I think perhaps the natural variability is too big for us to know in our lifetime. The warming over the past century, for which we have reasonably good data, is in the range of 0.5 - 1 °C, which is pretty much in the range of the undisputed warming of CO2 (i.e. with no forcing) plus natural variability.

On the environmental/political side of this, I'm not at all concerned about the planet as a whole. It can take a lot, and has endured warmer days before, and it seems to come out stronger. And humans are in a good position to adapt whichever way things turn. I'm rather concerned that environmentalists eager to adapt the climate and nature to our needs, to freeze things like it happened to be when human civilization came along. I thought that environmentalism was about having the adaptation to go the other way...?

On the environmental/political side of this, I'm not at all concerned about the planet as a whole. It can take a lot, and has endured warmer days before, and it seems to come out stronger. And humans are in a good position to adapt whichever way things turn. I'm rather concerned that environmentalists eager to adapt the climate and nature to our needs, to freeze things like it happened to be when human civilization came along. I thought that environmentalism was about having the adaptation to go the other way...?

It boils down to this: are humans altering the climate? Yes. Could this potentially have very negative consequences on human societies? Yes. Is there any good reason why we should not stop altering the climate? Make a case for it, but I don't see any. Ergo, we should stop altering the climate.

No one is arguing about whether Earth, or even life, will survive climate change. No one is arguing that extinction will be the result (at least no serious and informed person that I've heard). The argument is simply that it will cause a lot of trouble and hardship that we don't need, so we should just take the - honestly very easy - steps now to avoid encountering that possibility.

This experiment we're running with our environment provides us with virtually no benefit, and is potentially gravely detrimental. Even if the dissatisfying outcome were very unlikely, it would still be prudent to stop this climatological experiment.

HarbingerDawn, you worry too much. Global Warming in my opinion is similar to the quote in my signature. Even if there is Climate Change, in a Darwinian sense this is good as it forces adaptation = evolution.

"Religion is regarded by the common people as true - by the wise as false - and by the rulers as useful." Lucius Annaeus Seneca

in a Darwinian sense this is good as it forces adaptation = evolution.

I'm sure Bangladeshis dying in floods due to rising sea levels and third world people dying in droughts or potential future wars over water sources in desertifying areas will consider this a huge comfort.

Fortunately, I'm sure the wizard that makes the Earth grow will also cast a spell to prevent climate change.

apenpaap. That is right, adaptation always works via thinning of the herd. Most environmental issues are caused directly as a consequence of overpopulation. I suspect environmentalists see this as a good thing but never mention it as it would be bad for PR.

"Religion is regarded by the common people as true - by the wise as false - and by the rulers as useful." Lucius Annaeus Seneca

Thanks. Growing up, my interests were all over the place, but always involved natural science. I became fascinated in astronomy when I was 3, but was also interested in meteorology (I was hooked on the Weather Channel and wanted to be a storm chaser.) :P As I got older I studied a little bit of chemistry, mathematics, genetics, physics, and geology. But I obviously can't study everything. So I'm focusing on what interests me most -- astrophysics and planetary processes, particularly with exoplanets.

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I'm under the impression that many astrophysicists downplay the CO2-temperature link and put much faith in the sun/cosmic ray link. [etc]

Really? I find that rather surprising. I mean yeah, scientists do have a tendency to focus on their field more than others (naturally, we are human beings too!), but in my experience most astrophysicists simply don't have much knowledge on the subject (they're more interested in other things), so they trust to the expertise of those who do. Those that do say that the CO2/temperature link is much more important than the latter. Cosmic rays do appear to have an effect on cloud seeding, but the over impact on climate is more of a fine tuning, like solar activity.

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As I wrote previously, the number that is the answer to the sensitivity question is in my opinion less interesting scientifically than the reasons for that number.

I agree with you; science is about finding out why things happen the way they do, because this gives us insight. This is why scientists go into great detail figuring out the reasons for that number. This is why there are hundreds of papers per week on the various aspects of climate sensitivity. If you take the time to look through the current literature, it is incredible how specific some of these studies are.

Any good professor of physics will tell you that he/she cares more about how you arrive at a solution, than what the solution itself actually is.

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So even if the probability distribution actually nails it, we don't necessarily know the reasons. If you try to model something and it turns out that however you make the models, you get basically the same number out, then, yes, you get a really convincing probability distribution, but you will still have absolutely no clue what's going on in the physical world, since even false causes give the same result and you can't know that you picked the right model. [etc]

Okay, I understand your point but I don't think it is an accurate conclusion. Remember: although it is very difficult, we can disentangle and test each of the components that goes into climate sensitivity, and this kind of work is going on constantly. This is part of why climate science is such an active and extensive field. We absolutely do have a good, though by no means complete, handle of what is going on in the physical world. If we did not, then the agreement between different models and different physical evidence is an incredible coincidence with no rational explanation. If the actual climate sensitivity turns out to be very different than 3°C, we will be able to figure out where we were wrong. And if it's right on the money, there will be scientists who will continue to study the details because that's what good scientists do (and because they're probably interested in it).

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The geological aspect is interesting. There absolutely must be a beautiful thermostat. Otherwise we wouldn't have been around.

Oh yes, I find this extremely fascinating. I've had some incredible discussions with one of my professors about the "CO2 / rock-weathering thermostat" and its role in Snowball Earth episodes. If you're not familiar with it you might enjoy the read. Earth history is amazing; I wish I could learn all of it.

I gotta cut this post short here, but let me know if there's anything you'd like to continue discussing; I'm enjoying our talk.

Even if there is Climate Change, in a Darwinian sense this is good as it forces adaptation = evolution.

Since the adaptation would not be a result of genetics, this has nothing to do with biological evolution. Also, by your same logic, murder being legal would be a good thing, since it would force people to adapt and so those who could avoid being murdered or could murder all their potential killers would make the world stronger... right?

Your method of seeing the world is dangerous and foolish, and has led to enough trouble in human history already, including many of the issues we currently face.

HarbingerDawn, people with the genetic markers favourable to climate change will tend to survive, those without the genetic markers will tend to perish. It depends which attributes are favoured, it could be physical attributes that are able to withstand higher temperatures or it may be higher intellect that is selected where people build technological shelters to protect themselves from the elements. The naked body of man in many areas of the world is not fit for the environment, man must use clothing, tools and shelter with heating to survive, man does not have a thick covering of fur like most animals do, in the wild he must use his intellect and knowledge passed down from ancestors in order to survive. Higher man would never have evolved if the entire world was a garden of Eden.

I think animals may also become smarter, with man being the #1 predator of most large species, those in the wild will eventually adapt to avoiding traps and snares, fish in the ocean will develop a higher conciousness to avoid fishing nets and baited hooks. All life on earth is becoming more intelligent, the process of natural selection is a hard, long and painful process but most species, those which survive will eventually be better off for it.

I don't think what man is doing to the world is wrong, he is just following his natural instinct with is the same for all life. As long as the changes are slow and incremental, man will adapt to the changes and be better for it along with other life. The worse thing for life is a sudden change such as the dinosaur comet/sudden dinosaur earth expansion.

"Religion is regarded by the common people as true - by the wise as false - and by the rulers as useful." Lucius Annaeus Seneca

HarbingerDawn, you worry too much. Global Warming in my opinion is similar to the quote in my signature. Even if there is Climate Change, in a Darwinian sense this is good as it forces adaptation = evolution.

HarbingerDawn's concern is consistent with a rational and informed view of the world. You should try listening to him and others who have taken the time to educate themselves on these subjects.

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earth expansion.

Have you read nothing of what was told to you in the Conspiracy Thread, or are you just incredibly dense? Or are you intentionally trolling? I'm really beginning to wonder about you.

I think animals may also become smarter, with man being the #1 predator of most large species, those in the wild will eventually adapt to avoiding traps and snares, fish in the ocean will develop a higher conciousness to avoid fishing nets and baited hooks.

Please tell me you aren't serious about this

There is no evidence to support such claims, in fact the evidence suggest the contrary. A larger loss of life is happening due to the actions of mankind, both directly and indirectly. There is no "intelligence boom" that is happening. As a species we have grown far beyond many natural limitations and the farther we grow beyond natural selection the more of a threat we are to nature.

As of right now the only animals increasing in intelligence are lab rats that are genetically modified. Or animals like dogs that have been bred with such traits.

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I don't think what man is doing to the world is wrong, he is just following his natural instinct with is the same for all life.

Many of those natural instincts are part of the problem.

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As long as the changes are slow and incremental

The current changes happening are happening far too fast for most life to adapt.

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man will adapt to the changes and be better for it along with other life.

Evolution is not about improving anything. Evolution is simply about what works, and what works in one environment may end up hindering our survival in future environments.